The present invention relates to a laser distance measuring method for measuring the distance to a target by using laser light, and relates to an apparatus therefor. In particular, it relates to distance measurement using laser light modulated with a pseudo random signal.
The method of this type for measuring the absolute distance to a target by using laser light has been widely applied to the distance measurement in the field of civil engineering and construction industry. Some Examples are the distance measurement for the prevention of collision with moving substances, the distance measurement as visual information for remote control of robots, the distance measurement for examining the degree of damage or fatigue of refractories in the inside of a convertor or a pouring ladle used in an iron-manufacturing process, etc.
The distance measuring methods using laser light are generally classified into three, namely, a phase comparing method, a pulse modulating method, and a pseudo random signal modulating method. The outline of these methods will be described hereinbelow.
(1) Phase Comparing Method
As the phase comparing method, for example, there is such a method as disclosed in Japanese Patent Unexamined Publication No. Sho-62-75363. This method will be described hereinbelow with reference to FIG. 1.
Laser light emitted from a laser oscillator 21 is distributed into two light paths by a half mirror 27. Laser light on one light path enters into an acoustoopic modulator 22. The acoustooptic modulator 22 generates diffraction light having a frequency shifted by an interaction with a high-frequency signal from a high-frequency oscillator 23. The diffraction light is transmitted to a target 17 via a reflection mirror 44. A detector 43 monitors the diffraction light, that is, laser light. Light reflected on the target 17 enters into a heterodyne detector 40 through a half mirror 28.
On the other hand, laser light on the other light path provided from the half mirror 27 enters into the heterodyne detector 40 through the half mirror 28 to serve as reference light. The heterodyne detector 40 heterodyne-detects a differential frequency signal representing the frequency difference between the reflected wave from the target and the reference light from the half mirror 28. Because the phase of the differential frequency signal is delayed proportionally to the distance to the target, the distance to the target can be measured through the phase detector 42 which detects the phase difference between the reference signal from the high-frequency oscillator 23 and the differential frequency signal.
Because the phase comparing method employs a continuous wave sending system, reflected light from the target cannot be discriminated from unnecessary light, such as reflected light, leakage light, etc. from other places than the target, so that a large error may arise. In particular, when sensitivity is so high that weak reflected light can be detected, weak leakage light from an optical system related to radiation laser light may be detected by an optical system related to reception of light and may be superposed on the reflected light from the target to interfere therewith to thereby bring phase disorder, resulting in a large error.
(2) Pulse Modulating Method
For example, a pulse modulating method has been described in Japanese Patent Unexamined Publication No. Sho-58-76784. The method will be described hereinbelow with reference to FIG. 2.
Laser light from a laser oscillator 21 is distributed into two light paths by a half mirror 27. Light on one light path enters into an acoustooptic modulator 22 driven by a high-frequency oscillator 23. In the acoustooptic modulator 22, laser light is modulated to diffraction light having the frequency shifted. The diffraction light is passed through a slit 29, converged by a lens 54 and then enters into a light modulator 24. In the light modulator 24, the incident laser light is pulse-modulated with a pulse signal from a pulse generator 50. The pulse-modulated laser light is passed through a transmission optical system 11 and radiated toward a target 17. The reflected wave from the target 17 is passed through a reception optical system 12 and a half mirror 28 and enters into a light detector element 25.
Laser light on the other light path from the half mirror 27 is reflected on the half mirror 28 and then enters into the light detector element 25 so as to serve as reference light. The light detector element 25 heterodyne-detects the pulse-like reflected wave from the target 17 on the basis of the reference light to thereby generate a differential frequency signal pulse waveform representing the frequency difference between the reflected light and the reference light. The differential frequency signal pulse waveform is amplified by an intermediate-frequency amplifier 51 and then detected by a detector 52 so as to be a detection pulse waveform.
Because this detection pulse waveform is delayed by the propagation time of the laser light required for reciprocating the distance to the target, an information processing circuit 53 calculates the distance to the target 17 by measuring the time difference between a point of time when a pulse is generated by the pulse generator 50 and a point of time when the detection pulse waveform is obtained by the reception of the reflected light from the target 17.
Because the pulse modulating method uses an intermittent wave transmitting system, the reflected light from the target can be timely discriminated from unnecessary light reflected from obstacles even though such unnecessary light reflected from obstacles is present. In the pulse modulating method, however, not only is power of pulse-like light small on an average but the peak power of laser light to be transmitted is restricted because of problems of device configuration and safety. Accordingly, when the target is low in light reflectivity, the distance to the target cannot be measured. Therefore, when the target is low in light reflectivity, for example, a reflection tape having a large number of fine reflection substances on a surface thereof may be stuck to the target to increase the quantity of reflected light. However, the process for sticking such a reflection tape to the target is complex.
(3) Pseudo Random Signal Modulating Method
For example, a pseudo random signal modulating method has been described in Japanese Patent Unexamined Publication No. Sho-58-166281. This method will be described hereinbelow with reference to FIG. 3.
In a modulator 24, laser light generated by a laser oscillator 21 is subjected to intensity-modulation with a pseudo random signal generated by a pseudo random signal generator 60. The modulated laser light is radiated toward a target 17. Light reflected on the target 17 is received by a light detector element 25 and converted into an electric signal. Here, the waveform of the electric signal is stored in a high-speed storage device 62. In a delay correlation device 61, the correlation between the pseudo random signal generated by the pseudo random signal generator 60 and the reception pseudo random signal stored in the high-speed storage device 62 is calculated while delaying the pseudo random signal successively. The result of the correlation processing is given to a display recorder 63.
Because the phase of the pseudo random signal received as reflected light from the target 17 is delayed by the propagation time of light to the target 17, the correlation between the two signals is increased when a delay for the propagation time is given to the phase of the pseudo random signal generated by the pseudo random signal 60 so as to be used as a transmission signal. Accordingly, the distance to the target can be measured by measuring the delay time which maximizes the correlation.
Because the pseudo random modulating method uses a continuous wave transmitting system, the reflected wave from the target can be discriminated from the reflected wave from obstacles on the basis of the delay time for correlation processing even though such unnecessary reflected wave from obstacles is present. However, the pseudo random modulating method is complex in apparatus configuration and long in signal processing time. Specifically, the high-speed storage device and the delay correlation circuit may be constituted by analogue circuits or may be constituted by digital circuits. In the former case, the number of elements used is so large that the apparatus becomes complex in configuration. In the latter case, the operation processing is so complex that the processing speed becomes slow.